A multi-layer ceramic chip capacitor using ceramics as dielectric materials, which is mainly composed of the titanate such as barium titanate BaTiO.sub.3, having BaO to TiO.sub.2 mol ratio of 1, has become an essential component applied to current industrial and household electronics appliances due to its high-capacity in small size, fine high-frequency characteristics, i.e., electrical characteristics in high-frequency, excellent heat-resistance and adaptability for mass-production.
Since pure BaTiO.sub.3 is not exactly sufficient in dielectric constant under room temperature and dielectric characteristics such as thermal variation, the composition is typically modified by substituting Ca and/or Sr for a part of Ba, and Zr for Ti. With such a substitution, Curie temperature, i.e. transformation temperature of the crystal system at which the dielectric constant is exponentially improved, is shifted near to room temperature to improve its dielectric constant so that performance as a capacitor can be enhanced, in conjunction with its increased electrical resistance.
While there exists a single plate capacitor having each of the electrodes mounted on both sides of a platy dielectric material, a multi-layer capacitor is aboundingly fabricated, in which dielectric materials are thinned and then the materials and counter electrodes are alternately laminated to enlarge an effective electrode area. Such a multi-layer ceramic chip capacitor is fabricated in a process comprising steps of: first printing or applying a paste, which is composed of conductive metal powder to be served as an internal electrode, on a green ceramic sheet thinned, then laminating a number of such sheets and sintering to integrate them, and mounting an external electrode, which is connected with the internal electrode, outside the integrated body.
For making a dielectric constant of ceramics adequately high, it is generally required to sinter ceramics at a high temperature of approximate 1300 degrees centigrade (.degree. C.) When a part of oxygen in the ceramics is dispersed during this treatment, the dielectric material would become semi-conducting material to result in lower insulation performance, which is insufficient for a capacitor. Thus the sintering is usually carried out under oxidizing atmosphere including oxygen.
When a melting point of the internal electrode is relatively low, there is the possibility to melt and run out during sintering. Therefore, it is required for the internal electrode to be composed of a metal, whose melting point is higher than the sintering temperature for the ceramics, being able to be sintered at the same temperature as that of the ceramics and being not oxidized under an oxidizing atmosphere at such temperature. Pt, Pd and alloy of them have conventionally been applied as a metal for the internal electrode meeting this requirement. The more the number of laminated layers is increased with thinning of the thickness of a ceramic layer, the more the weight ratio of these materials to the entire capacitor weight is increased. In this case, the cost of the capacitor is considerably increased due to the expensive cost of these materials. Therefore, in view of small specific resistivity, high melting point and low cost, Ni and Ni alloy are applied as the electrode material.
At high sintering temperature under oxidizing atmosphere, Ni is oxidized to annul functions in the electrode and further it runs into the ceramics in the form of oxide to lose its performance as a capacitor. In the sintering atmosphere, when its oxygen partial pressure is lowered or hydrogen is included thereinto, in order to prevent oxidization of Ni, there causes a problem that the ceramics would be deoxidized to be turned to semi-conducting material. Thus, a continuing development and practical application have been carried on for getting at ceramics having adequate deoxidization resistance, with which the ceramics is not turned to semi-conducting material and its insulation resistivity is not decreased even if it is sintered under the reducing atmosphere or the atmosphere with low oxygen partial pressure, and also satisfactory characteristics for the dielectric material.
U.S. Pat. No. 3,920,781 discloses an invention in which the mol ratio of barium titanate ceramics between a large radius cation (divalent ion such as Ba and Ca) and a small radius cation (quadrivalence ion such as Ti and Zr) is adapted to be above 0.95 but no more than 1.0. This invention describes that in cases where base metal, such as Ni, is applied to an electrode and sintering at 1000 to 1400.degree. C. is conducted under an atmosphere with low oxygen partial pressure, an equal or better electric characteristic can be obtained as compared with that of a capacitor in which a platinum electrode sintered in the air is applied.
Thereafter, as an invention disclosed in Japanese Patent Publication No. 57-42588, it was shown that a mol ratio of BaO group oxides substituting Ca or Sr for a part of Ba and having a large radius cation to TiO.sub.2 having a small radius cation is adapted to be 1.005 to 1.030 or more than 1 so that reduction resistant ceramics can be obtained. Nowadays, a ceramic dielectric material having deoxidization resistance with such mol ratio is commonly applied to an electrode using base metal such as Ni.
Though the aforementioned improvement for a ceramic dielectric material has enabled base metal, such as Ni, to be applied to the internal electrode, there has remained problems of less stability in characteristics and lower reliability as compared with that of a multi-layer capacitor in which Pd or the like is applied to the electrode.
In relation to the above problem, various improvements have been proposed not only for enhancement of reliability but also for enhancement of performance and further lowering of sintering temperature. For example, Japanese Patent Laid-Open Publication No. 3-133116 discloses a multi-layer ceramic chip capacitor comprising: a dielectric material composed of ceramics, in which a mol ratio between an oxide substituting Ca or Sr for a part of Ba and an oxide substituting Zr for a part of Ti is adapted to be 1.00 to 1.020 and also 0.005 to 0.5% by weight of at least one component selected from the group consisting of oxides of Y, Gd, Tb, Dy, Zr, V, Mo, Zn, Cd, Tl, Sn and P is added, and an internal electrode composed of Ni or Ni alloy. This invention describes that in the case where oxides of Y and such is added, insulation resistivity life is improved to a large extent as compared with the case without such additive. As a similar improvement, Japanese Patent Laid-Open Publication No. 4-367559 discloses a ceramics dielectric material having deoxidization resistance, in which each of Mn or Cr oxide, Si oxide and Y oxide is added respectively with 0.005 to 1.0% by weight to a composition having slightly over 1.0 mol ratio of BaO group oxides to TiO.sub.2 group oxides. Further, Japanese Patent Laid-Open Publication No. 6-45182 and Japanese Patent Laid-Open Publication No. 6-275459 disclose inventions for extending the life by adding V or W oxide, SiO.sub.2, Al.sub.2 O.sub.3 and so on in addition to oxides of Y and Mn.
Japanese Patent No. 2521855, Japanese Patent No. 2521856 and Japanese Patent No. 2521857 disclose inventions for a multi-layer ceramic chip capacitor consisting of a main component in which a mol ratio of BaO group oxides substituting Ca and Mg for a part of Ba is slightly larger than a mol ratio of TiO.sub.2 group oxide partially substituting Zr, to which 0.002 to 0.04 mol of oxides of rare earth elements are added, and subsidiary components of 0.2 to 5% by weight composed of compositions with low melting point, such as Li.sub.2 O--SiO.sub.2 -alkaline earth metal oxide and B.sub.2 O.sub.3 --SiO.sub.2 -alkaline earth metal oxide.
Furthermore, Japanese Patent Laid-Open Publication No. 5-217793, Japanese Patent Laid-Open Publication No. 5-217794 and Japanese Patent Laid-Open Publication No. 5-217798 disclose an invention including the same effect as described above, in which Ca is applied as a substitute for a part of Ba. In the approximately same composition, an invention for a capacitor having a dielectric material, wherein Sr is added in addition to Ca and Mg as substitute elements for a part of Ba, is disclosed in Japanese Patent Laid-Open Publication No. 6-69065.
As above, to enable a low cost base metal to be applied to an internal electrode of a ceramic capacitor, a great deal of research and development has been carried out to establish a ceramic dielectric material having high dielectric constant and being capable of avoiding deterioration of its characteristics even in cases where sintering for its integration is conducted under reducing atmosphere and further at lower temperature. However, in a multi-layer ceramic chip capacitor, strong demand for large capacity with small size, enhancement of performance and reliability and also reduction of manufacturing costs continuously remain.